1. Field of the Invention
The present invention generally relates to hearing aids, and more particularly to devices which improve upon conventional hearing aids by using modulation techniques adapted to the characteristics of auditory and vestibular hearing.
2. Background Description
The traditional hearing aid is an air-conduction amplifying system such that a microphone picks up air conduction sounds, amplifies them and present them in the ear canals as an air conduction signal to the ear drum. These type of devices offer a small frequency range and also offer a small dynamic range of intensity.
Bone conduction hearing aids have also been developed for users where the conventional hearing aid is not satisfactory. A bone conduction device is attached to the head of the user and the output from a microphone pick-up is amplified and fed into this device which causes bone vibration. These devices operate over a small dynamic range and are designed principally for individuals whose middle ears could not be surgically repaired or for very young children who have abnormalities of the middle ear that cannot be surgically repaired until they are older. These bone conduction devices currently are rarely used.
The present invention is based in part on technology described in U.S. Pat. No. 4,982,434 to Lenhardt et al. (“Lenhardt, 1991”). That technology involves transposing air conduction sounds in the conventional or audiometric range which is a frequency range of about 100 to about 10,000 Hertz. These frequencies are shifted into the supersonic range which are frequencies above 20 kHz to about 108 kHz or higher and then transmit these supersonic frequencies by bone conduction or the like to the human sensory system. The hearing aid may transpose air conduction sound from the speech frequencies to the supersonic ranges in such a fashion that noise burst frequency modulated signals and quiet bursts that relate to speech frequencies will be shifted into the supersonic range. These signals are delivered by a bone conduction attachment such as a high fidelity electrical to vibrator transducer, preferably a piezoelectric type, functionally connected for bone conduction in the head.
It is hypothesized that the hearing aid and method described in Lenhardt 1991 is based on a system of hearing quite distinct from normal hearing based on air conduction. It utilizes bone conduction and parallels the primary hearing response of reptiles. In reptiles, there is no air conduction hearing, but hearing is mediated via the saccule which, in man, has been considered an organ responsible for balance and determining acceleration and movement. In reptiles, this organ is a hearing instrument and it possesses hearing potential in amphibia and in fish as well.
Phylogenetically, in evolution, hearing in fish, amphibia and reptiles is mediated by vibratory frequencies that work through vestibular systems. In amphibia, both bone and air conducted frequencies impinge on vestibular receptors. In reptiles, air conduction hearing is non-existent unless transduced via skin or bone to the vestibular saccule which is the primary hearing organ, as the cochlea does not exist. During evolution, as mammals evolved from reptiles, therapsids or amphibia, as gait, posture and skull evolved, so did the mammalian and avian cochlea which took over the role of the saccule as the primary hearing organ. The internal ear, or cochlea is now the primary mammalian acoustic contact with the external environment. The saccule, although equipped with the neuro-cortical functional capacity to ascertain sound became a back-up system of limited value, except for balance and motion detection. The awareness of the vestibular developmental role in evolutionary biology of hearing, was lost as physiologists expanded on our understanding of the role of air conduction with clinical emphasis on the physiology and pathology of the cochlea. Otolaryngologists, audiometrists, speech therapists, psychologists and physiologists look upon the saccule and utricular systems as accelerometers or motion detectors. The residual role of the saccule and vestibule in hearing perception is lost to current knowledge.
The hearing aid technology described in Lenhardt, 1991 is believed to utilize direct bone transmission to the saccule and this enables hearing to be maintained via a system independent of air conduction and the inner ear although integrated with the air conduction system. This provides a mechanism for allowing the nerve deaf to hear, but in addition, provides an alternative source of informational transfer independent of sounds moving through air. The sound is transmitted directly to the bones of the skull, and utilizes frequencies that are perceived by the saccule and not by the inner ear.
An advantage to utilization of the vestibule (saccule) as a hearing organ is that its response is transmitted via the vestibular nerve which can substitute for, or augment communication in, a damaged acoustic nerve. This is important in aging because of the relative longer functional life of the vestibular nerve in aging. The vestibular nerve also provides an alternative to acoustic nerve injury that is of value in the sensory/neural deaf.
If hearing is viewed from a physical perspective, the cochlea is a collection of receptors linked to a mechanical device that matches the impedance of sound in air with that of sound in the cochlear fluid. If this cochlear transformer or transducer was not present most of the sound energy would be reflected away from the head. In contrast to the air mediated response of the cochlea, the otolithic organs in the vestibule, the saccule and utricle, respond to acceleration or body movement and inertial forces. The cochlea responds to sound pressure in similar fashion to a microphone while the saccule acts as an accelerometer which measures sound (vibration) in a solid medium.
The cochlea is sensitive to audiometric frequencies primarily in the range of 100 to approximately 10,000 Hertz. But the most important frequencies for a spoken voice are from 500 to 2500 Hertz. In the supersonic bone conduction technology described in Lenhardt, 1991 these frequencies are amplified and converted to a higher frequency. The frequency conversion or transposition shifts the frequency up from a normal audiometric range to the supersonic range which is above 20,000 Hertz and extends to approximately the 100,000 Hertz range. This transformation function may be linear, logarithmic, a power function or a combination of these and may be customized for each individual. To improve the recognition of the sounds being heard, the waveform may be modified by the waveform modification or signal processor. The supersonic signal may be modified to optimize the intelligibility of the signal. However, even without the waveform modification, the signal has a substantial intelligibility.
The supersonic bone conduction technology uses a transducer to apply the supersonic signals as supersonic vibrations to the skull, preferably at the mastoid interface. The transducer provides such vibrations at a frequency in the supersonic range and preferably from above 20,000 Hertz to approximately 100,000 Hertz. These frequencies are perceived as frequencies within a normal audiometric range by the brain and permit an intelligible understanding of what is being heard in the audiometric range even though the brain receives the signals primarily at supersonic frequencies. This is a key element of the prior art technology described in Lenhardt, 1991. Even though the frequencies are shifted to supersonic vibration frequencies they can still be interpreted by the brain as speech at audiometric frequencies.
The waveform modification may also include filters for certain bands which may have to be amplified further or some bands may have to be attenuated depending on how the signal is multiplied for customizing the hearing aid to the user. Customizing is not absolutely essential but can be used to improve the perceptual signal to the user so that it is a smooth speech perception that is balanced for the best perception.
Frequently, in voices, the low frequency will come in with the most intensity so low frequencies would in some cases be attenuated. Those frequencies that are critical for speech detection (500 to 2500 Hz) may be preferentially amplified. The signals can be cleaned to improve the speech perception by lumping some frequencies such as frequencies below 500 Hertz together and attenuating them. But the critical frequencies for voice communication between 500 Hertz and 2500 Hertz may be resolved so that small differences between the frequencies can be detected and discerned. The just noticeable differences (JND) of pitch varies at different frequencies generally in accordance with the 10% rule at supersonic frequencies. Pitch discrimination of young subjects show that at a tone of 2,000 Hertz, the JND is approximately 2 Hertz and at 15,000 Hertz the JND is approximately 150 Hertz. When the tone is 35,000 Hertz the JND is approximately 4,000 Hertz and at 40,000 Hertz the JND is 4500 Hertz. Thus, the 10% rule is that the JND is approximately 10% of the frequency of the tone and this extends into the supersonic region. So in addition to bunching or lumping together the low frequencies below 500 Hertz, the most important frequencies of 500 Hertz to 2500 Hertz are expanded when converted to supersonic frequencies so that the small differences in the frequencies can still be discerned under the 10% rule. Through bone conduction, the vibration frequencies in the supersonic range are perceived by the brain as the original audiometric frequencies. These signals can be modified to customize them to the individual subject and the transducer being used. This may be done through a combination of attenuation of some of the frequencies, a great amplification of some of the other frequencies and by wave shaping of the signal.
The state-of-the-art in noise control for hearing devices is active noise cancellation, which is effective for high frequencies, but ineffective for low frequencies and broad band noise. Most military operations occur in low frequency, broad band ambient noise. At present there is no good communication system for operation in a 120 dBA noise environment.
Conventional wisdom places the frequency range of human hearing between 20 Hz and 20 kHz. The upper limit is governed by the response of the basilar membrane with a center frequency of 20 kHz in the basal region. However, this region of the basilar membrane is capable of sensing frequencies up to 90 kHz or so with sufficient excitation. We refer to the 20 kHz-90 kHz ultrasonic band as the “quiet channel” of the auditory system. It has been shown (Lenhardt, 1991) that speech can still be recognized with 85% intelligibility when the frequencies are shifted into this range. Additionally, it is very difficult to mask these frequencies since the ambient noise ceiling is typically low. With sufficient power even deaf listeners can discriminate speech modulated by ultra-sound (i.e. acoustic energy between 10 and 100 kHz) at a level of 40% correct. Ultrasonic hearing aids based on this finding are commercially available.
Ultrasound is audible either by bone or fluid conduction to the inner ear. The most efficient transfer path for ultrasound is with the transducer (actuator) interface on the skin over the mastoid bone, or the skin of the neck or side of the face over the massitor muscle. Detection is unlikely since ultrasound is not audible by air conduction (up to about 145 dB) unless there is some intermediary substrate or fluid coupling.